Foraging Strategies and the Avoidance of Predation by an Aquatic Insect, Notonecta Hoffmanni

Field and laboratory experiments demonstrated that the degree to which an aquatic insect, Notonecta hoffmanni, shows avoidance behavior towards an important predator is propor- tional to the magnitude of the risk of predation. Laboratory experiments showed that I, II, and III instar juvenile N. hoffmanni can suffer significant mortality due to predation from adult N. hoffmanni whereas IV and V instars suffered no mortality from adults. In particular, the relative risk of predation from adults was I > II > III > IV = V - no risk. Based on these data I predicted that I, II, and III instars should avoid adults whereas IV and V instars should not, and that the relative degree of avoidance should be I > II > III > IV = V = no avoidance. Two measures of predator avoidance were documented: alterations in the use of space, and reductions in movement. In both the laboratory and the field the effects of adults on the behavior of juvenile notonectids were determined by comparing the juvenile's behavior when adults were experimentally removed to that in controls where adults were present. Alterations in spatial patterns were as predicted, i.e., in both the laboratory and the field only I, II, and III instars avoided adults by altering their use of space, and the degree to which spatial patterns were altered was highly correlated with the relative risk of predation. In the laboratory, reductions in movement were also as predicted, i.e., only I, II, and III instars reduced their movement in the presence of adults and the reduction in movement was highly correlated with the risk of predation. However, in the field, only I and III instars avoided adults by moving less; i.e., II instars did not conform to expectations. Accordingly, in the field the correlation between the reduction in movement and the risk of predation was positive but not signif- icant. In the laboratory, the effects of adults on juvenile feeding rates were also documented. Because I and II instars avoided adults by altering their spatial and movement patterns, they suffered a cost in reduced feeding rates. Unexpectedly, III instars altered their feeding behaviors but suffered no reduction in feeding rates. Partial correlation analysis showed that a decrease in the juvenile's use of the central region of water bodies (where prey were four times more dense) resulted in a significant decrease in their feeding rates, but that a reduction in their movement had no such effect. I then discuss the effects on the predictions of optimal foraging theory of some common mecha- nisms by which notonectids avoid their predators. Because the degree of avoidance is related to the risk of predation I also discuss the possibility that feeding behavior represents an adaptive compromise between the conflicting demands of feeding efficiently and avoiding predators. Finally, because the avoidance of predation reduces both the feeding rate of predators on foragers and of foragers on prey, I examine the effects of such reductions on population interactions among predators, foragers, and prey.